holography

The principle of holography. (A) To form a hologram,
an object is illuminated in coherent light (usually from a laser).
A photographic plate is then exposed in a combination of the light
scattered from the object and some of he original beam of coherent
light (the reference beam) reflected in a mirror. (B) To view the
hologram, it is illuminated in coherent light similar to the original
reference beam. The light scattered from the hologram then affords
views of an image of the original object in whatever orientation the
hologram is viewed. Holography depends on interference in the plane
of the hologram between the light scattered by the object and the
coherent reference beam.

Holography is a system of recording light or other waves
on a photographic plate or other medium in such a way as to allow a three-dimensional
reconstruction of the scene giving rise to the waves, in which the observer
can actually see around objects by moving his or her head. The resulting
plate, or hologram, records the interference
pattern between waves reflected by the scene and a direct reference
wave at an angle to it; it is view by illuminating it from behind and looking
through rather than at it. The high spatial coherence needed prevented exploitation
of the technique, originated in 1948 by the Hungarian-born British physicist
Dennis Gabor (1900–1979), until the advent of lasers.

There are two kinds of hologram: reflection, in which the 3D image seems
to be behind the plate; and projection, in which the image stands out like
a mirage.

Holography is increasingly used in measurement and inspection (e.g., for
irradiated nuclear fuel rods either out of or in a working reactor, and,
for security coding on bank cards) and for decorative or artistic artifacts.
Color holograms are possible, and three-dimensional television may ultimately
be feasible.

A hologram obtained by using apparatus so that laser light reflected from the objects travels about the same distance as the reference laser beam reflected onto the photographic plate by a mirror.